Hippocampal cortex from guinea pigs was studied, in vitro, by electrophysiological techniques. Slices of hippocampus approximately 0.3-0.4 mM thick permit visualization of neuronal cell layers and synaptic pathways. Various synaptic pathways have been identified by electrical stimulation and by recording in neuronal cell layers in different regions of the hippocampus. Changes in ionic composition of the superfusing solution have been used to characterize the synaptic responses. These synaptic pathways are being used to study the nature of the synaptic transmitter released and the interaction of psychotrophic and other centrally acting drugs. In addition, a long-term potentiation of synaptic transmission is also the subject of current investigation.